Recently, demand has increased for electricity generated from alternative sources other than fossil fuels and nuclear sources. A common source of alternative energy is wind driven turbines having rotor blades rotatable by the wind. The most common type of wind turbine for generating electricity is a horizontal axis wind turbine (“HAWT”) in which the main rotor shaft for the generator rotates about a horizontal axis. The rotor blades extend radially from the horizontally oriented rotor shaft and rotate in a generally vertical plane oriented generally transverse to the wind direction. In an “upwind” configuration, the rotor blades are positioned upwind of the generator and the turbine tower. An inherent requirement of upwind turbines is that the rotor blades have minimal aeroelastic deflection such that the rotor blades do not deflect during rotation and strike the turbine tower as each rotor blade passes the turbine tower. Accordingly, the rotor blades for upwind turbines are typically reinforced to minimize deflection during rotation. However, the structural reinforcement increases the material requirements and the weight of the rotor blades, which decreases the efficiency of the turbine as well as increasing the cost of rotor blades for upwind turbines. The drawbacks of minimal aeroelastic rotor blades required for upwind turbines are exaggerated as the length of rotor blades have increased to provide the necessary scale required by the increased demand the electricity from wind energy. In particular, as the length of rotor blades have increased to extreme scales of over 100 m in length, the reinforcement required for upwind rotors of extreme scale turbines to minimize aeroelasticity over the entire length the rotor blades can be cost prohibitive or substantially hinder the efficiency of the wind turbine.
An alternative turbine type is a “downwind” type turbine in which the rotor blades are positioned downwind of the generator and the turbine tower when the horizontal axis is oriented parallel to the wind direction. In this arrangement, the rotor blades can be more flexible than the minimally aeroelastic rotor blades of upwind type turbine as the downwind rotor blades can flex away from the turbine tower as the rotor blades rotate about the rotor shaft. Accordingly, downwind rotor blades can be lighter and less expensive than upwind rotor blades of similar lengths. A drawback of downwind turbines not present in upwind turbines is that the downwind positioning of the rotor blades causes the rotor blades to pass through the wake created by the turbine tower and the generator. The wake momentarily slows each rotor blade as the rotor blades pass through the wake. In an addition, the wake created by the turbine tower temporarily applies aerodynamic loading to the passing rotor blades causing the rotor blades to flutter with each rotation through the wake. The fluttering of the rotor blades fatigues the rotor blade over time shortening the effective lifespan of the rotor blade as compared to similarly sized upstream rotor blades. Accordingly, the reduced initial cost of the downwind rotor blades is offset by the increased maintenance costs to repair and maintain downwind turbines.